1. Field of the Invention
The present invention relates to an apparatus for forming glass elements such as glass lenses and prisms and, more particularly, to an apparatus for forming glass elements which forms glass elements by interposing a glass material between a pair of dies, which are engageable with each other and can open and close, heating the dies and glass material, and pressing the glass material.
2. Description of the Related Art
Methods for manufacturing glass elements such as glass lenses that are required to provide high accuracy are broadly divided into two. One is to form a molten glass material to a shape close to that of a final formed article and to grind and polish this formed article thereby to finish it. In the other method, a glass material formed to the weight and shape corresponding to a final formed article is heated and finished by performing reheat pressing by use of precision dies.
In the former manufacturing method by grinding and polishing, a dozen or so steps are required for forming curved surfaces. In addition, large amounts of glass grinding dust harmful to workers are generated. Moreover, this manufacturing method has another disadvantage that it is difficult to manufacture high-value added glass elements having optical surfaces of aspherical surface shape with the same accuracy and in large quantities.
In contrast, in the latter manufacturing method by reheat pressing, a glass element is formed by printing the shape of dies onto a glass material. For this reason, only one step of press forming is necessary for forming curved surfaces and, at the same time, it becomes possible to carry out manufacturing in a clean environment. Furthermore, this manufacturing method has another advantage that once dies are fabricated, large quantities of glass elements can be manufactured according to the accuracy of the dies.
Incidentally, when usual optical glass elements are formed by reheat pressing, the forming temperature is 700° C. at the highest and cemented carbide alloys are generally used as die materials. Therefore, in terms of thermal expansion, materials having coefficients of linear expansion close to those of cemented carbide alloys or ceramics are used in bolts which fasten the above-described dies and heat insulating cylinders, which adjoin the dies and are usually made of ceramics. Usually, cemented carbide alloys, tungsten alloys, etc. are used.
However, in the case of glass materials of high melting point such as silica glass, which requires a forming temperature of about 1400° C., if fastening bolts of the same material as in the forming of usual optical glass elements are used, looseness would occur in the bolts. Furthermore, in terms of heat resistance, the use of such fastening bolts had the problem that strength decreases extremely, resulting in an early breakdown.
In an apparatus for forming glass elements, in order to prevent heat emitted from top and bottom die assemblies from transferring to a moving shaft and a fixed shaft, heat insulating cylinders made of ceramics of low thermal conductivity are interposed between the top and bottom die assemblies and these shafts. When a usual optical glass is formed, the forming temperature is about 700° C. at the highest and heat is sufficiently insulated by interposing heat insulating cylinders made of ceramics. However, in the case of glass materials of high melting point such as silica glass, which requires a forming temperature of about 1400° C., the heat insulating cylinders are heated together with the dies during the heating of the glass materials and this had the problem that the durability of the heat insulating cylinders decreases or the heat insulating cylinders are broken down.
In addition, when glass materials are heated by use of an infrared lamp, it is necessary to cool the infrared lamp itself in order to protect the infrared lamp from the heat generation from the infrared lamp itself. For this purpose, it is effective to thermally isolate the top and bottom die assemblies and the infrared lamp from each other by arranging the infrared lamp outside a formation chamber in which the top and bottom die assemblies etc. are housed. In order to arrange the infrared lamp outside the formation chamber, it is necessary that the formation chamber allow the transmission of infrared rays, and conventionally, silica glass of high infrared transmittivity is used.
Moreover, in order to prevent the oxidation of dies made of metal etc., it is necessary to eliminate oxygen near the dies. And there has been adopted a method which involves confining the dies and glass material within the formation chamber and purging oxygen by filling the formation chamber with an inert gas or a method which involves evacuating the interior of the formation chamber.
However, it is difficult to fabricate the whole formation chamber from silica glass. Therefore, the formation chamber is designed in such a manner that a side surface portion from which the top and bottom die assemblies and glass material are irradiated with infrared rays is covered with a silica glass tube and other portions from which irradiation with infrared rays is not performed are fabricated from metal etc. and that in order to increase the sealing performance of the formation chamber, the space between the silica glass and the metal is sealed with O-rings.
In the case of forming of glass materials which require high forming temperatures such as silica glass, the forming temperatures are as high as 1350° C. to 1600° C. or so and the silica glass tube is partially heated to temperatures of not less than 500° C. or so even when the silica glass tube is air cooled. Therefore, due to the heat transfer from the silica glass tube, the O-rings in contact with the silica glass tube are also heated. Furthermore, because infrared rays reach the O-rings while reflecting within the silica glass, the O-rings are also heated even when they are installed in places remote from the infrared lamp. For this reason, it is necessary to use expensive heat resistant materials as O-ring materials. However, even in this case, there was a problem of short life and frequent replacement, which is unfavorable in terms of cost.